Switch mode power supplies or switching regulators, also referred to as DC-to-DC converters, are often used to convert an input supply voltage to a desired output voltage. A switching regulator provides power supply function through low loss components such as capacitors, inductors, and transformers, and power switches that are turned on and off to transfer energy from the input to the output in discrete packets. A feedback control circuit is used to regulate the energy transfer to maintain a constant output voltage within the desired load limits of the circuit.
A switching regulator can be configured to step up the input voltage or step down the input voltage or both. Specifically, a buck switching regulator, also called a “buck converter,” steps down the input voltage while a boost switching regulator, also called a “boost converter,” steps up the input voltage. A buck-boost switching regulator, or “buck-boost converter,” provides both step-up and step-down functions.
The operation of the switching regulator is well known and is generalized as follows. A power switch is turned on to apply energy to an inductor to allow the current through the inductor to build up. When the power switch is turned off, the voltage across the inductor reverses and energy is transferred to an output capacitor and the load. A relatively constant output voltage is maintained by the output capacitor.
A problem faced by most types of switching regulators is maintaining high efficiency at light load conditions. Switching regulators can become very inefficient at light load (low output current) conditions. One solution to improve efficiency at light load conditions is to use Pulse Frequency Modulation (PFM) in the feedback control loop. When the PFM technique is applied, the switching frequency of the main power switch is made a function of the output power. By switching less often during low output current, the switching losses also reduce at light load.
FIG. 1 is a schematic diagram of a conventional boost switching regulator using PFM feedback control. Referring to FIG. 1, a boost regulator 10 includes a PFM control circuit 24 coupled to drive a first power switch M1 and a second power switch M2. The boost regulator 10 further includes an inductor L1 and an output capacitor COUT. The boost regulator 10 receives an input voltage VIN on a node 12 and provides an output voltage VOUT having a substantially constant magnitude on a node 16 for driving a load 35. The output voltage VOUT is fed back to a feedback node 18 of the boost regulator 10 to form a feedback control loop to realize regulation and control of the output voltage. In the PFM feedback control loop, the output voltage may be fed back to the feedback node directly or through a resistor divider. In the present illustration, a resistor divider circuit including resistors R1 and R2 divides down the output voltage VOUT to provide a feedback voltage VFB (on node 18) to the PFM feedback control loop.
More specifically, the feedback voltage VFB is coupled to an error comparator 20 to be compared with a reference voltage VREF to generate an error signal VERR (node 22) having logical high and low values. The error signal VERR is then provided to the PFM control circuit 24 to generate the control signals NGATE (node 28) and PGATE (node 32) for driving the first power switch M1 and the second power switch M2. The NGATE and PGATE signals may be buffered by buffers 26 and 30 respectively. The control signals NGATE and PGATE are operative to turn power switch M1 and M2 on and off alternately so that a switching voltage VSW is generated at the switching node 14. In particular, when the PFM control circuit 24 drives the power switch M1 to turn on and drives the power switch M2 to turn off, inductor L1 has the input voltage VIN impressed upon it, and the current through the inductor builds up. When the PFM control circuit 24 drives the power switch M1 to turn off and drives the power switch M2 to turn on, the voltage across the inductor L1 reverses (“fly back”) and inductor L1 delivers energy through switch M2 to the output capacitor COUT and the load 35. The output capacitor COUT filters the ramping inductor current to generate a substantially constant output voltage VOUT at the output node 16.
For a PFM feedback control loop or a hysteretic control loop to operate correctly and reliably, a certain amount of voltage ripple is required by the feedback control loop. More specifically, because of the switching action at the power switches, all switch-mode regulators generate an output current ripple through the switched inductor L1. This current ripple manifests itself as an output voltage ripple principally due to the equivalent series resistance (ESR) in the output capacitor COUT placed in parallel with the load. The ESR of the output capacitor COUT is denoted as a resistor RESR in FIG. 1. Recently, low ESR capacitors are preferred to realize improved efficiency and low output voltage ripple in switching regulators. However, the low ESR capacitors do not generate sufficient output voltage ripple for meaningful feedback control. The desire for low output voltage ripple at the output voltage contradicts with the PFM feedback control loop requirement of a certain amount of ripple for reliable operation. The low output ripple signal (typically less than 1 mV) is often too small to be meaningfully used by the PFM feedback control loop of the switching regulator.
In particular, when the low ripple signal on the output voltage is further divided down by the resistor divider and presented to the feedback control loop to be sensed by the error comparator, the ripple signal at the feedback node can be so small that any noise in the switching regulator can completely swamp out the available ripple signal. In operation, the error comparator that senses the feedback voltage can be easily triggered due to noise rather than being triggered by the actual ripple signal. Noise-triggered events at the error comparator often result in multi-pulsing or chattering on the switching node and also cause undesirable frequency components.
Ripple injection circuits to introduce a ripple signal in the feedback loop have been described for buck switching regulators. For example, U.S. Pat. Nos. 7,482,791 and 7,482,793 illustrate examples of ripple injection circuits that can be applied in buck regulators using fixed on-time control.